The present invention, in some embodiments thereof, relates to a method for estimating interference between signals in a multi-channel communication system, more particularly, but not exclusively, to estimating interference in point-to-point multi-transmitter wireless communication systems.
Estimating the interference is typically used for canceling the interference and cleaning up the received signals.
The background of the invention will be described with examples from a wireless point to point communication system, which illustrates some of the problems which the present invention solves.
Fixed wireless point to point communication systems are common, for example in cellular networks backhaul and private networks in campuses. Such systems are characterized by high RF frequencies, ranging from several GHz to tens of GHz. As RF frequencies go higher, an associated phase noise increases as well. In single carrier modulations, such as QAM, tracking phase noise may be done using phase-locked loops (PLLs), pilot symbols, or a combination of both. PLL techniques have been known for tens of years. A central component of PLL circuits is a phase detector. Its function is to remove modulated data from a carrier signal and estimate an instantaneous phase of the carrier signal. Removing the data requires estimating the transmitted data. When there are errors in the estimated data, it is not removed correctly, and carrier phase tracking may loose lock, generating more decision errors. This approach falls into the category of decision-directed methods.
In order to overcome the problem of decision errors, it has been proposed to insert known data into the transmitted stream in the form of pilot symbols. US patent application 2005/0111603 of Ginesi et al proposes a practical way to do it for single carrier systems. Once every several data symbols, a pilot symbol is inserted to the data stream. In a receiver, additive noise is filtered out, and an instantaneous phase of the pilot symbol is estimated and interpolated. The above-mentioned approach falls into a category of data-aided methods, and generally outperforms decision-directed methods. A cost of the approach is in form of bandwidth dedicated for the pilot symbols. Naturally, data aided methods and decision-directed methods may be combined.
In addition to tracking carrier phase, efforts are made to increase spectral efficiency of such wireless communication systems. An approach for increasing the spectral efficiency is by transmitting in two orthogonal polarizations. A polarization is the axis along which the electric field of the signal oscillates as radio waves propagate. It is possible to transmit two separate signals, one with the electric field oscillating in one plane, typically named the horizontal plane, and one with the electric field oscillating in a perpendicular plane, typically named the vertical plane. We name these polarizations H and V polarizations correspondingly. There are other ways to share space between two orthogonal polarizations, and the present invention relates to all of them. Communication systems employing two polarizations are called dual-polarization systems, and they double the spectral efficiency of single polarization systems.
Reference is now made to FIG. 1, which is a simplified block diagram illustration of an example prior art dual polarization communication system.
The dual polarization communication system 100 includes a main polarization transmitter 101 and a cross polarization transmitter 102.
The two transmitters 101 102 transmit two data streams TH 103 and TV 104, using two carrier signals 105 106, in two orthogonal polarizations. For purpose of simplifying description, an identical carrier frequency ω is described for both carrier signals 105 106. Phase noise associated with the two carrier signals 105 106 is denoted by φH and φV correspondingly. The two data streams, TH 103 and TV 104, are “mixed” with their respective carrier signals 105 106, and optionally amplified by amplifiers 117A 117B, thereby producing two carrier signals for transmission 103A 104A, which are transmitted to two receivers 110 111.
Due to imperfect isolation between polarizations, for example in antennas 107 108 113 114 and in a propagation channel 109, the two carrier signals for transmission 103A 104A often mix. Received signals, RH 115 and RV 116, are often mixtures of their intended H and V signals with some amount g 120 of interfering signals V and H correspondingly. Generally speaking, g 120 may be viewed as a filter, that is, a vector of coefficients which multiplies the interfering signals. In order to simplify explanation, g is described as a single coefficient which multiplies the interfering signals. Additive thermal noise nV 121 and nH 122 is also typically present.
The received signals RH 115 and RV 116 are therefore described by Equation 1 below:RH=(TH·ejφH+g·TV·ejφV+nH)·ejωt RV=(TV·ejφV+g·TH·ejφH+nV)·ejωt  Equation 1
Equation 1 shows that a received signal, whether RH 115 or RV 116, includes a component of a cross polarized signal, RV 116 or RH 115 respectively.
When one wishes to estimate the cross polarization, typically used for canceling the interference and cleaning up the received signals, typically the cross-polarization component in a main signal is “buried” under the main polarization signal. The cross-polarization component is multiplied by the coefficient g, as indicated by the equation above. In some cases g is so small, for example g<0.1, that it is difficult to determine the phase of the cross-polarization component.
Background art includes:
US patent application 2005/0111603 of Ginesi et al;
U.S. Pat. No. 7,046,753 to Resheff et al;
Stuber, G. L., Barry, J. R, Mclaughliln, S. W., Li, Y., Ingram, M. A. and Pratt, T. G., “Broadband MIMO-OFDM Wireless Communications”, Proceedings of the IEEE, Vol. 92, No. 2, pp. 271-294, February 2004; and
Jiang, M., Hanzo, L., “Multiuser MIMO-OFDM for Next-Generation Wireless Systems”, Proceedings of the IEEE, Vol. 95, No. 7, pp. 1430-1469, July 2007.